1. Field of the Invention
The present invention relates to an image forming apparatus which adjusts a toner density in accordance with humidity, and an image forming method.
2. Description of the Related Art
Conventionally, there has been a widely known image forming apparatus adopting an electrophotographic method, in which an image is formed on a recording sheet by performing an exposing operation to a photoconductive drum whose surface is uniformly charged with electricity based on a predetermined image data to form an electrostatic latent image on the surface of the photoconductive drum, forming a toner image by a developing device which develops an electrostatic latent image by charged toner particles, and directly or indirectly transferring the toner image from the photoconductive drum to the recording sheet and fixing the toner image.
In such image forming apparatus, a developing device stores developer particles including carrier and toner particles. The image density is increased by increasing a ratio of toner particles included in the developer i.e. the toner density, and the image density is decreased by lowering the toner density. However, the amount of electric charge to toner particles is changed in accordance with the humidity. When the humidity is high, the amount of electric charge is lowered. Accordingly, toner particles become easily separated from carrier particles, and the density of an image is increased. On the other hand, when the humidity is low, the amount of electric charge is increased so that toner particles become hardly separated from carrier particles, and the density of an image is decreased.
Thus, there has been a known image forming apparatus (for example, refer to Japanese Unexamined Patent Publication No. 2000-47438) which decreases the toner density of a developing device to decrease the density of an image when the humidity is high, and increases the toner density of the developing device to increase the density of an image when the humidity is low, so that the change of image density due to the change of humidity can be adjusted.
Meanwhile, in the above-described image forming apparatus, there has been a disadvantage that an appropriate image density cannot be obtained and the image quality is degraded when a humidity in a power-off state and a humidity in a power-on state are greatly different from each other. For example, it is taken in consideration the case where a power source of the image forming apparatus is turned off under an environment where an air conditioner is turned on in summer, and the power source of the image forming apparatus is turned on next day while the air conditioner is not turned on. In such a case, the toner density in the developing device is increased since the humidity is low when the power source of the image forming apparatus is turned off on the previous day. Then, on the next day, when the power source of the image forming apparatus is turned on, toner particles with high density at the time when the power source is turned off on the previous day remain in the developing device even though the humidity is high. At this time, even if it is tried to adjust the image density by decreasing the toner density, the image density is not adjusted until all of the toner particles with high density remained in the developing device are used. Therefore, there existed a disadvantage that an appropriate image density cannot be obtained and the image quality is degraded.